In order to achieve effective operational performance, sensitive instruments often require a stable operational environment. For instance, to achieve maximum resolution in a state-of-the-art high resolution transmission electron microscope (HREM), a stable environment must be available at the microscope site. Due to the charge state of the electron beam and the use of electromagnetic lenses and coils, the HREM is sensitive to fluctuations in its magnetic field environment especially in the area of the HREM's objective lens. A typical value for maximum allowable fluctuation in the ambient magnetic field environment is .+-.1 mGauss (10.sup.4 Gauss=1 Tesla=1 Weber/m.sup.2 : Earth's magnetic field=500 mGauss). Fluctuations should be minimized in order to perform maximum resolution imaging experiments with a HREM and other magnetically sensitive instruments.
In general, magnetic environments contain horizontal and vertical magnetic field components. With respect to a HREM, instabilities in these components can have the effect of causing an uncertainty in the location and angle of the electron beam with respect to the optic axis (induced beam tilt), or can cause undesirable modification of the lens focusing condition in the instrument. Therefore, magnetic environment instabilities may substantially degrade the performance of a HREM or other sensitive instrument, unless the magnetic environment is controlled.
A horizontally-oriented magnetic field (e.g. transverse to the optic axis) acts as a magnetic deflector and tends to induce a tilt of the electron beam. A stable field of this type may be corrected through the use of beam deflectors during the normal alignment procedure of the microscope. A vertically-oriented field acts as an electromagnetic lens over the entire length of the microscope column and modifies the vertical magnetic field inside a lens polepiece (e.g. equivalent to a modification of the focal length and therefore the focusing condition of the lens). In a stable magnetic environment, the presence of the field is automatically compensated by adjusting the focusing strength of each lens and manipulating the specimen height within the objective lens polepiece. However, it is not generally practical nor efficient to compensate for unstable magnetic environments by adjusting the focusing strength or by manipulating the specimen height.
Magnetic field instabilities pose a significant problem to the operational effectiveness of magnetically sensitive instruments, since in the minimum case they tend to introduce uncertainty in the operating parameters of the instrument, and in the worst case tend to prevent any reasonable results from being obtained. The present invention provides a convenient and economical solution to the problem. For the purpose of description focus will be on an unstable vertical magnetic field around a JEOL, Ltd. JEM4000EX HREM, although the solution is more generally applicable to horizontal instabilities and other sensitive instruments.
When the time period of a magnetic field component becomes relatively large (e.g. earth's field), it can be considered as a stable DC magnetic field. A stable DC magnetic field is not a concern because it is typically compensated for by the instrument's adjustment settings. In contrast, when a magnetic field component's period is larger than the operational period of the sensitive instrument, for instance, the exposure time for a single image taken by a HREM, but not large enough to be considered a stable DC field, it is defined as a slowly wandering DC magnetic field. A slowly wandering DC field component could arise, for example, from an electrically-powered railway running in the vicinity of the instrument and may significantly degrade instrument performance. An AC field is any magnetic field component whose period is equal to or shorter than the operational period of the instrument (e.g. single image exposure time of a HREM).
Magnetic fields which are generated by the instrument itself are another significant problem in providing the stability required for state-of-the-art magnetically sensitive instruments used in applications, such as, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Electron Beam Microprobe Instrumentation, Electron Beam Lithography, High Spatial Resolution Auger Electron Spectroscopy (AES) and High Spatial Resolution Electron Energy Loss Spectroscopy (EELS). These instruments will be referred to as magnetically active instruments.
A magnetically sensitive instrument is located within a given magnetic environment. Such environment may contain slowly wandering DC magnetic field components and AC magnetic field components. These components may be oriented in a vertical or horizontal direction with respect to the magnetically sensitive instrument. The compensation system detects these magnetic components at the magnetically sensitive instrument and then generates equal and opposite magnetic field components to control the environment. When the magnetically sensitive instrument is a magnetically active instrument it generates magnetic field components during its operation. The controlled magnetic environment will be referred to as the primary magnetic field. The magnetic field components generated by the instrument are secondary magnetic field components. The components detected at the instrument will be referred to as the detected magnetic field.
One solution to stabilize the magnetic environment at the sensitive instrument is to use magnetic shielding to reduce the effect of an unstable magnetic environment for measurements of low-level magnetic fields. Patton and Fitch (1962). This type of solution is not generally effective due to inherent difficulties such as the size of the instrument, convenience for instrument maintenance, and removal of instrument generated ambient heat.
Others have ventured to solve these problems by using a detection means to determine the magnetic field components in the magnetic environment at the site of the instrument and then provide neutralizing magnetic fields through an electromagnetic compensation coil. Several variations on this approach have been attempted in connection with other magnetically sensitive instrument installations. However, these attempts did not provide the level of performance that is required in order to perform high resolution electron microscopy. While operation of many state-of-the-art sensitive instruments including the HREM require stabilization of better than 1 mGauss, Gemperle and Novak (1976) and Gemperle et al. (1974) disclose compensation systems only providing stabilization to within 3 mGauss and Hadley et al. (1971) discloses a compensation system only capable of stabilizing the magnetic environment to between 1-2 mGauss. Furthermore, these compensation techniques and others could not compensate for both slowly wandering DC and AC magnetic field components. For instance, Buncick (1982), provided a compensation system with a very slow response time only capable of stabilizing DC magnetic field components. Whereas, Gemperle and Novak (1976), Gemperle, et al. (1974), and Hadley, et al. (1971) developed compensation systems only capable of detecting the AC magnetic field component and therefore, only compensated for AC magnetic fields. Compensation for both AC and DC magnetic field instabilities was provided by Hand (1976) but such result was accomplished only by simultaneously using two compensation systems.
Additionally, none of the prior art compensation systems can provide compensation for effective operation of magnetically active instruments. The magnetic fields generated by the instrument itself during its operation are problematic for compensation systems in the prior art because these systems would neutralize these field components thereby opposing fields created by the instrument necessary for its operation. For example, during operation of a HREM, the user changes the lens currents to maximize HREM focussing. Such changes in the HREM's electromagnetic lenses would be detected together with the components detected by the compensation system. For effective use of the HREM it is undesirable to have the compensation system opposing the focussing of the lenses.
Since a vertically oriented field will often require stabilization over the entire length of the instrument, practical restraints on the size and orientation will often be imposed on the coil. For example, a vertical magnetic field component acts as an electromagnetic lens over the entire length of a HREM column. Since the instrument is likely to be located in a laboratory of fixed size, certain electromagnetic coil orientations are more practical than others. One method to produce a uniform vertical magnetic field in a room is to construct a solenoid with length much greater than its diameter. This solution renders the room inaccessible from a human engineering standpoint. A single horizontal coil can also produce a vertical magnetic field at its center. This approach suffers from the necessity to locate the coil such that the objective lens polepiece is centered and in the plane of the coil, again raising human engineering concerns. A single coil located out of the objective lens horizontal plane requires a significantly larger current in order to accurately compensate for the ambient magnetic fields so that the coil will induce a large Vertical magnetic field gradient along the microscope column.
A system which overcomes all the problems associated with the prior art by providing compensation to both DC and AC magnetic field components while not interfering with secondary field components generated by a magnetically active instrument, must also fulfill practical constraints such as size and energy requirements.